The present invention relates to portable machines such as drilling machines and relates more particularly to an electric drilling machine having a plurality of speeds of rotation of the spindle.
In order to ensure that a drilling machine is adaptable to various drilling diameters in different materials such as wood, metal, concrete or other materials, it is desirable to provide it with regulating means providing a plurality of speeds of its chuck.
Machines of this type are known which are provided with an electronic speed variator.
These machines, which usually comprise a speed reducer having two ratios associated with the motor driving the spindle and controlled by a manually operated ratio changing means, are liable to operate under adverse conditions of speed and torque in that the electronic variator controls the output speed of the spindle without taking into account the mechanical ratio engaged.
Drilling machines having an electronic speed variator are known which comprise instantaneous display means for the speed of rotation of the machine
However, such a display only gives an indication of the conditions under which the machine is employed without suggesting to the user the optimum conditions of use bearing in mind the material worked upon and the section of the holes to be drilled.
Consequently, there is often wear of the drill due to the use of the machine at unsuitable speeds of rotation.
Further, the motor of the machine may be subjected to excessive over-heating which is liable to produce serious breakdowns.